1. Field of the Invention
The present invention relates to ink-jet hard copy apparatus and, more particularly, to the art of generating control signals for firing ink droplets from a scanning ink-jet printhead and, specifically to methods and apparatus for compensating for variations in printhead-to-media spacing and printhead scanning velocity.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
FIG. 1 depicts an ink-jet hard copy apparatus, in this exemplary embodiment a computer peripheral printer, 101. A housing 103 encloses the electrical and mechanical operating mechanisms of the printer 101. Operation is administrated by an electronic controller (usually a microprocessor-controlled printed circuit board, not shown) connected by appropriate cabling to a computer (not shown). Cut-sheet print media 105, loaded by the end-user onto an input tray 107, is fed by a suitable paper-path transport mechanism (not shown) to an internal printing station where graphical images or alphanumeric text is created. A carriage 109, mounted on a slider 111, scans the print medium. An encoder strip 113 and appurtenant devices (not shown) are provided for keeping track of the position of the carriage 109 at any given time. The fundamentals of encoder tracking are set out in U.S. Pat. Nos. 4,786,803 and 4,789,874 (Majette, et al.) (assigned to the common assignee hereof and incorporated herein by reference in their entireties). A set 115 of ink-jet pens, or print cartridges, 117A-117D are releasable mounted in the carriage 109 for easy access. In pen-type hard copy apparatus, separate, replaceable or refillable, ink reservoirs (not shown) are located within the housing 103 and appropriately coupled to the pen set 115 via ink conduits (not shown). Once a printed page is completed, the print medium is ejected onto an output tray 119.
An ink-jet pen includes a printhead which consists of a number of columns of ink nozzles. The columns of nozzles fire ink droplets that are used to create a print column of dots on an adjacently positioned print media as the pen is scanned across the media. A given nozzle of the printhead is used to address a given vertical column position, referred to as a picture element, or xe2x80x9cpixel,xe2x80x9d on the print media. Horizontal positions on the print media are addressed by repeatedly firing a given nozzle as the pen is scanned. Thus, a single sweep scan of the pen can print a swath of dots. The print media is stepped to permit a series of scans. Dot matrix manipulation is used to form alphanumeric characters, graphical images, and even photographic reproductions from the ink drops. Generally, the pen scanning axis is referred to as the x-axis, the print media transport axis is referred to as the y-axis, and the ink drop firing direction is referred to as the z-axis.
Note that when a nozzle is fired, the ink is ejected from the pen at a finite velocity and it must travel a finite distance along the z-axis between the pen and the print media (for convenience and without limitation to the scope of the invention, the word xe2x80x9cpaperxe2x80x9d will be used hereinafter to mean any form of print media). Since the pen is not stopped at each position during scanning in the x-axis, a fired ink droplet will also have a velocity in the x-axis direction as it traverses the distance to the paper surface. Thus, in order to hit a target pixel, any given nozzle should be fired a finite time before the pen positions the nozzle directly over the location where the dot is intended to be printed. However, in the art it is often generally assumed that all drops will have the same offset and thus, without such time of drop firing compensation, overall print quality is not affected even though the image is shifted as a whole. If at all compensated, an average advanced time of the firing signal is calculated by using the expected flight time of the drop and the current pen velocity, each of which is known from the design of a specific implementation of ink-jet hard copy apparatus (e.g., it is known that the maximum allowable carriage speed without print quality degradation is calculated by taking the time it takes for pen control logic circuitry to shift one set of data up to the pen and fire divided by the pen nozzle stagger distance (explained hereinbelow); the flight time is calculated by dividing the nozzle-to-paper spacing by the velocity of the ink drop.
A typical prior art drop firing encoder is shown in FIG. 1A with a timing diagram therefor shown in FIG. 1B. An encoder 113 provides two output timing signals, xe2x80x9cEncAxe2x80x9d and xe2x80x9cEncB,xe2x80x9d which are decoded 121 as fundamental coarse position indicators of where the carriage 109 is during a scan. The leading and trailing edge of each encoder signal can thus be used in conjunction with a counter 122 to generate carriage position, tracking carriage movement in units such as {fraction (1/150)}th inch (this value will be used throughout as an exemplary embodiment herein; no limitation on the scope of the invention is intended thereby nor should any be inferred therefrom). A series of fire timing pulses, xe2x80x9cFTPxe2x80x9d_COUNT, is generated for each position signal, allowing the FIRE pulse actually to trigger firing of a plurality of nozzles in the printhead. Fire timing pulses are generated continuously by a clock during normal printing and used in accordance with the number of nozzles arrays in a particular printhead design as needed. The Fire Position circuitry 123 combines the position information with a value for a nozzle firing register 123 to generate a nozzle firing pulse, xe2x80x9cFIRE,xe2x80x9d e.g., every period comprising movement of the carriage {fraction (1/150)}th inch. The leading or trailing edge is also used in a Period_Counter 124 to determine the carriage velocity. Dividing 125 the period by a predetermined number (e.g., 100, taken from an extrapolation_division register (not shown)xe2x80x94a value related to the number of nozzle firing desired per period for a particular printhead implementation, the FTP_COUNT pulses) provides an extrapolation for the timing of the FTP_COUNT pulses. That is, an extrapolator latch 126_counter 127 takes the measurement of the carriage period as measured in clock cycles divided by the value kept in the extrapolation-division register. The FTP_COUNT pulses are also provided 128 as fine position indicator for carriage position.
However, the horizontal distance from the actual advanced firing position of a given nozzle to where the drop actually lands is dependent on the scanning velocity of the pen. Knowing the total flight time of the ink drop and the pen scan velocity, the distance can be calculated by multiplying these two values. If the scan velocity of the pen is constant, the amount by which the firing signal precedes each pixel position is a constant. As discussed above, in this case the whole printed image is just shifted by a constant amount; that is, the image is moved by the number of dot positions that equal the over-shoot distance. Compensation in the foregoing manner moves the whole rendered image to attempt to compensate simply for this error. However, this does methodology does nothing to improve instantaneous drop placement accuracy within each scan swath.
In fact, when a pen is scanned across the paper, its velocity is not constant. Also, there are pen acceleration and deceleration ramps at each end of a scan which may still be within the intended printing zone on the paper. Firing nozzles during such changing pen velocity causes successive ink drops to land at varying distances from the intended uniform spacing. Furthermore, in order to increase throughput and to improve print quality by using print modes such as checkerboarding the printed pixels"" dot matrix pattern on the paper, bi-directional printing is often the preferred print mode. Note also that bi-directional scanning prints pixels in opposite time-of-firing directions, further complicating the pixel alignment. In other words, a trade-off must be made between throughput and image quality in accordance with deciding when to fire ink drops using current fire pulse timing solutions.
Another solution is to make the sweep width wider than the printed area so as not to print on the acceleration and deceleration ramps of a scan but only during supposed constant pen velocity periods; this causes both a throughput penalty and requirement for a larger apparatus workspace footprint.
Moreover, a further problem exists when the nozzle-to-paper spacing is not a constant. The variation in this nozzle-to-paper spacing causes the drop positioning to change non-uniformly across the width of the scan. Therefore, drop positioning will change across the page, causing drops not to hit the intended address pixel grid correctly. Thus, there is a need to calculate the firing advance dynamically to remove positioning errors which would result from changes in the nozzle-to-paper spacing during any one scan.
A further time-of-firing complication is added when a vertical column of nozzles on the printhead is broken into groups, called xe2x80x9cprimitives,xe2x80x9d generally for use with different color inks being fired from a single printhead. In order to prevent having to fire all nozzles simultaneously, within a column and within a primitive, the nozzles are staggered horizontally in the pen scan x-axis direction by an amount slightly less than the space between print columns divided by the number of nozzles per primitive. This means that the firing from one nozzle to the next occurs at a defined spacing, known as the xe2x80x9cstagger distance,xe2x80x9d (or simply xe2x80x9cstaggerxe2x80x9d) which is less than the spacing between dots on the media. The carriage must move this stagger distance between firing different nozzles of the same column (e.g., stagger time is calculated taking the time it takes the carriage to traverse the {fraction (1/150)}th inch and dividing this time by the number of stagger distances in that {fraction (1/150)}th inch). In this manner, the nozzles of each primitive can fire sequentially to create a vertical column of dots on the paper.
In order to solve these problems, there is a need for dynamically varying the ink drop fire timing as a function of pen velocity. Note that this compensation for flight time assumes pen-to-paper spacing is constant and a static flight-time value can be used when performing pen velocity compensation, while at the same time, the variation in this spacing causes the drop positioning to change across the width of the paper since the pen velocity compensation is being performed statically when a dynamic flight-time may be needed. Thus, there is a need for compensation of both factors in order to deposit ink droplets accurately on intended target pixels.
In its basic aspects, the present invention provides an ink drop fire timing control device for an ink-jet hard copy mechanism for producing dot matrix printing on print media, the hard copy mechanism including an ink-jet pen and a carriage for scanning the pen across print media along a linear axis. The device includes comprising: a mechanism for generating periodic carriage position signals as the carriage is scanning the pen across print media along a linear axis; a mechanism for producing ink drop fire timing signals based upon the periodic carriage position signals; and a flight compensation mechanism for extrapolating a value representative of expected ink drop flight time error from the pen to the print media and advancing the ink drop fire timing signals to compensate for the expected ink drop flight time error such that ink drop flight time is compensated for velocity changes of the carriage as the carriage traverses the linear axis, wherein scanning position interrupt signals are generated by comparing carriage position with a next predetermined interrupt position.
In another basic aspect, the present invention provides an ink drop fire timing control device for an ink-jet hard copy mechanism for producing dot matrix printing on print media, the hard copy mechanism including an ink-jet pen, a carriage for scanning the pen across print media along a linear axis, and mechanism for generating periodic carriage position signals representative of periodic predetermined pen scanning positions along the axis as the carriage is scanning the pen across print media along a linear axis. The timing control device includes: paper shape compensation mechanism for generating a value representative of expected flight time for each of the periodic predetermined pen scanning positions along the axis calculated from a predetermined paper shape profile; and a mechanism for adjusting ink drop fire timing based on the value representative of expected flight time such that ink drops are ejected from the pen before the carriage positions the pen at a position for firing based on the signals representative of periodic predetermined pen scanning positions along the axis.
In another basic aspect, the present invention provides an ink drop fire timing control method for an ink-jet hard copy mechanism for producing dot matrix printing on target pixels of a print media, the hard copy mechanism including an ink-jet pen having a printhead with a plurality of ink drop firing nozzles arrayed as a staggered vertical column, a carriage for scanning the pen across print media along a linear horizontal axis, and mechanism for generating periodic carriage position signals representative of periodic predetermined pen scanning positions along the axis as the carriage is scanning the pen across print media along a linear axis. The method includes the steps of:
providing a signal indicative of coarse position of the carriage during scanning;
from the indicative of coarse position, deriving a periodic ink drop firing time signal;
from the signal indicative of coarse position, extrapolating a signal indicative of fine position of the carriage during scanning, the fine position being a predetermined subdivision of the coarse position by a number equal to the plurality of ink drop firing nozzles;
providing a signal indicative of expected flight time of a drop from the printhead to the print media;
from the signal indicative of fine position and the signal indicative of expected flight time, deriving a flight time error signal; and
from the flight time error signal, advancing the periodic ink drop firing time signal such that ink drops are fired before the carriage is positioned over a target pixel.
The step of providing a signal indicative of expected flight time of a drop from the printhead to the print media includes the steps of:
programming a paper profile value for each the fine position; and
incrementing the expected flight time when the profile value indicates pen-to-paper spacing is increasing at a fine position along the axis and decrementing the expected flight time when the profile value indicates pen-to-paper spacing is decreasing at a fine position along the axis.
In still another basic aspect, the present invention provides an ink-jet paper shape compensation device for generating a value representative of expected flight time for each of the periodic predetermined pen scanning positions along the axis. The paper shape compensation device includes: a re-loadable down counter mechanism for counting at each of the periodic predetermined pen scanning positions along the axis; and connected to the counter mechanism, mechanism for changing the value representative of expected flight time such that the value representative of expected flight time is incremented when pen-to-paper spacing is increasing and decremented when pen-to-paper spacing is decreasing at each of the periodic predetermined pen scanning positions along the axis.
It is an advantage of the present invention that it improves the ink drop positioning accuracy across a print medium scan by compensating for the change in ink drop flight time during velocity fluctuations and during carriage velocity ramps.
It is an advantage of the present invention that it allows ink drop flight time changes to be implemented as a simple, adjustable, incrementer/decrementer circuit.
It is an advantage of the present invention that it provides compensation for the change in ink drop flight time during variations of printhead-to-paper distance across a print medium scan.
It is another advantage of the present invention that it provides printhead-to-paper distance variation and scanning velocity variation compensation for bi-directional ink-jet printing.
It is still a further advantage of the present invention that it automatically compensates during carriage acceleration and deceleration velocity ramps, allowing a wider print zone than constant velocity printing modes.
It is yet a further advantage of the present invention that accurate printing during velocity ramps allows a narrower carriage travel and permits a smaller workplace footprint for a hard copy apparatus.
Other objects, features and advantages of the present invention will become apparent upon consideration of the following explanation and the accompanying drawings, in which like reference designations represent like features throughout the drawings.